Plants form relationships with organisms in the soil, including bacteria and fungi (1, 2). Belowground relationships between plants and associated microorganisms may be mutualistic, commensal, or antagonistic (3), and are driven by complex chemical signalling mechanisms (4, 5). Strigolactone signalling is an example of one such mechanism and has been previously reported to be involved in interactions between arbuscular mycorrhizal fungi and N-fixing Rhizobia, such as Rhizobium spp., with plant roots, but how strigolactones are detected by either fungi or Rhizobia is currently unknown (4-7).
Strigolactones, as well as the related compounds karrikins, belong to a class of chemicals which contain a butenolide moiety (8, 9). These can function as chemical stimuli within plants, influencing their gene expression. Notable plant responses to butenolides include promoting germination due to smoke exposure (10, 11), increased root length in response to low soil phosphate levels and facilitating root and shoot growth (12). Both strigolactones and karrikins are perceived by the α/β-hydrolase enzymes D14 and KAI2 (13) respectively. An additional α/β-hydrolase, RsbQ, identified in Bacillus subtilis and its relatives but also found in distantly related species including Rhizobium leguminosarum (14), bears significant sequence and structural resemblance to KAI2 and D14. This suggests a shared evolutionary origin and potential functional likeness among these three proteins. We have determined that B. subtilis RsbQ can interact with strigolactone analogues (15) and are now extending this to the RsbQ-like α/β-hydrolase, to determine if this is the receptor involved in the pea-R. leguminosarum symbiosis. We have been able to show via differential scanning fluorimetry and intact protein mass spectrometry that this protein interacts with the strigolactone analogue GR24. Future work will include solving the crystal structure of this protein and biological assays to determine symbiosis phenotype with pea and wild type or knockout mutant Rhizobium leguminosarum.